Source is where something comes from or begins. When you research a topic for a school project, your sources are the books, websites, or articles where you found your information. When you trace a river to its source, you're finding the spring or lake where the river first begins to flow.

The word appears in many contexts. A historian might study primary sources like letters and diaries written by people who lived through historical events, rather than secondary sources written by later historians. A journalist protects her sources by not revealing who gave her information. Scientists cite their sources to show where their data came from.

Source can also be a verb: to source ingredients means to find them and choose where they come from. A restaurant might source its vegetables from local farms.

When teachers ask you to “cite your sources,” they want to know where you learned what you're writing about. This matters because it shows you didn't just make things up, it lets readers check the information themselves, and it gives credit to whoever did the original work. Understanding sources helps you tell the difference between reliable information and rumors, between facts someone can verify and opinions someone just made up.